One commonly used system for data display utilizes a device such as a cathode ray tube wherein a beam of electrons is intensity modulated while raster scanning the phosphor coated face of the tube. Other data display systems may employ a laser or other light source as the scanning beam and a xerographic drum or photographic film as the recording medium. Each of these display systems is capable of displaying graphics or alphanumeric type data either by generating adjacent lines of varying length or, more commonly by assembling arrays of rows and dots or pixels to form a pixel matrix.
Some types of prior art display systems are capable of simultaneously displaying graphics and alphanumeric data; typically, the graphics data comprises conventional television signals representing an image to be displayed and the alphanuneric characters are superimposed, by masking techniques, on the television image. Normally, the alphanumeric characters to be displayed are produced by a character generator which includes a memory having digitally coded words corresponding to each character which may be displayed.
Display systems of the type mentioned above have found an important use in computerized and numerical control systems for controlling machine tools and the like. The operator of the machine tool inputs and receives data to the machine control system via a keyboard and CRT type display. In this context, it is important that the operator be able to clearly read the display, even from a distance of six feet or more, and that he follows the proper sequence of instructions displayed on the screen.
Prior art display systems are less than satisfactory when used for CNC and similar applications. For example, characters are normally surrounded by graphics information thereby making the characters difficult to read even at a close distance. Moreover, the size of the characters is fixed, consequently, even if large character fields are provided to allow distant viewing, the display system may not be easily modified to provide smaller character fields (and thus additional character lines) when the display is viewed from a short distance, as during operator set-up of a CNC machine tool.
Another problem associated with prior art display systems is that of flicker or interference of the displayed image when the operator changes the characters to be displayed. This arises from the fact that the video output from the character generator section is interrupted when the central processing unit of the system is reading from, or writing into, the character generator's memory.
Known display systems which simultaneously display graphics and character type information or symbols may employ separate mamories for respectively storing graphics and alphanumeric data prior to display thereof. The outputs of these memories are "summed" in order to generate a composite video signal used to scan the display screen. The approach of summing the graphics and alphanumeric data signals, however, results in substantial nonuniformity of the displayed image. For example, identical characters may be displayed with differing intensities on various parts of the screen because of the nature of the particular graphics data superimposed on such characters; this, of course, makes viewing somewhat more difficult.
Accordingly, it is an important object of the present invention to provide a device implemented method for simultaneously displaying graphics and alphanumeric data on a display, such as a CRT screen, which eliminates each of the deficiencies inherent in prior art displays mentioned above.